The invention relates to a plasma picture screen provided with a front plate which comprises a glass plate on which a dielectric layer and a protective layer are provided, with a carrier plate provided with a phosphor layer, with a ribbed structure which subdivides the space between the front plate and the carrier plate into gas-filled plasma cells, and with one or several electrode arrays on the front plate and the carrier plate for generating electrical corona discharges in the plasma cells.
Plasma picture screens can generate color pictures with high resolution, large screen diagonal, and have a compact construction. A plasma picture screen comprises a hermetically closed glass cell which is filled with a gas, with electrodes arranged in a grid. The application of a voltage causes a gas discharge which generates mainly light in the vacuum ultraviolet range. This VUV light is converted into visible light by phosphors and emitted through the front plate of the glass cell to the viewer.
In principle, two types of AC plasma picture screens can be distinguished: a matrix arrangement and a coplanar arrangement of the electrode arrays. In the matrix is arrangement, the gas discharge is ignited and maintained at the intersection of two electrodes, one on the front plate and one on the carrier plate. In the coplanar arrangement, the gas discharge is maintained between the electrodes on the front plate and ignited at the intersection point with an electrode, a so-called address electrode, on the carrier plate. The address electrode in this case lies under the phosphor layer.
The luminance of a plasma picture screen depends on several factors: 1. with what efficacy VUV light is generated in the plasma discharge; 2. with what efficacy the phosphors are excited by the VUV light; 3. with what efficacy the phosphors convert VUV light into visible light; and 4. with what efficacy the visible light from the plasma cell reaches the viewer.
In a coplanar arrangement of the electrode arrays, half of the VUV light generated in the gas discharge reaches the front plate, where it is absorbed by the layers present there. This effect is even further increased for part of the VUV light because the VUV light is re-absorbed in the gas space in that gas atoms are excited from their ground state to a higher energy level. The light is indeed emitted again subsequently, but it is diverted from its original direction, so that also light which had originally been directed towards the phosphor layer can reach the front plate. This reduces the efficacy with which the phosphors are excited by the VUV light.
The phosphor layer must be sufficiently thick and dense so that the VUV light emitted in the direction of the carrier plate is absorbed as fully as possible by the phosphor layer and converted into visible light. VUV photons which are not absorbed by the phosphor layer will reach the carrier plate and are absorbed therein. To prevent this, comparatively thick phosphor layers are used, or the particle size of the phosphors is reduced, which leads to a decrease in the VUV transmission for a given layer thickness. It is disadvantageous, however, that the efficacy of the phosphors is reduced in proportion as the particle diameter becomes smaller, and in particular the blue-emitting phosphors show an increased degradation.
The invention has for its object to provide a plasma picture screen with an improved luminance.
This object is achieved by means of a plasma picture screen provided with a front plate which comprises a glass plate on which a dielectric layer and a protective layer are provided, with a carrier plate provided with a phosphor layer, with a ribbed structure which subdivides the space between the front plate and the carrier plate into gas-filled plasma cells, and with one or several electrode arrays on the front plate and the carrier plate for generating electrical corona discharges in the plasma cells, and provided with an UV light emitting layer.
It is particularly preferred that the UV light emitting layer is provided on the protective layer.
The VUV light originating from the plasma discharge and emitted in the direction of the front plate is converted into UV light in the UV light emitting layer. The UV light is emitted towards the phosphor layer, where it is converted into visible light. Since the VUV photons emitted in the direction of the front plate are not absorbed there, but are converted into UV photons, substantially more photons will excite the phosphors on the carrier plate.
It is also preferred that the UV light emitting layer is provided between the carrier plate and the phosphor layer.
Transmitted VUV photons are not absorbed by the carrier plate, but are converted into UV photons by the UV light emitting layer. These UV photons are emitted in the direction of the phosphor layer and thus excite the phosphors.
It is particularly preferred that the UV light emitting layer emits UV-C light.
The photodegradation of phosphors by VUV light, for example of Eu2+-activated phosphors in the phosphor layer, can be prevented by irradiation with UV-C light.
It is preferred that the UV light emitting layer comprises a VUV phosphor with a host lattice chosen from the group of alumninates, borates, fluorides, oxides, phosphates, and sulfates.
These host lattices are efficient VUV phosphor lattices because they have a great bandgap.
It is furthermore preferred that the UV light emitting layer comprises a VUV phosphor which is activated by Pb2+, Ce3+, Pr3+, or Bi3+.
These heavy metal ions are suitable activators for VUV phosphors.
It is furthermore preferred that the VUV phosphor in the UV emitting layer is chosen from the group SrAl12O19:Ce, LaPO4:Ce, CeMgAl11O19:Ce, LuBO3:Pr, GdBO3:Pr, LaBO3:Pr, YBO3:Pr, LaPO4:Pr, YPO4:Pr, LuPO4:Pr, LaB3O6:Pr, SrSiO3:Pb, MgSO4:Pb, CaSO4:Pb, SrSO4:Pb, (Ca,Mg)SO4:Pb, (Sr,Ca)SO4:Pb, CaLi2SiO4:Pb, Ba(Y,Gd,Lu)B9O16:Bi, YF3:Bi, YOF:Bi, Y3Al5O12:Bi and (Gd,La)B3O6:Bi.
All these VUV phosphors show a small Stokes shift, i.e. the energy level distance between the excitation and emission bands is small, so that these phosphors emit UV light and no visible light.
It is preferred in particular that the VUV phosphor in the UV light emitting layer is LaPO4:Pr.
LaPO4:Pr enhances the luminance of a plasma picture screen in a particularly efficient manner, because it has a high quantum efficiency "PHgr"xe2x89xa780%.
It is preferred in particular that the particles of the VUV phosphor are coated with MgO.
A coating of MgO acts as a stabilizing protective covering which reduces the photodegradation of the VUV phosphors. MgO forms a hard layer insoluble in water on the VUV phosphor particles, it does not react with the VUV phosphor, and is itself not degraded by radiation. The magnesium oxide is intrinsically colorless, so it does not affect the color value of the VUV phosphor.
In an advantageous embodiment, a UV light reflecting layer is provided on the front plate.
This layer has the purpose of reflecting UV light, which is emitted in the direction of the front plate, towards the phosphors.
It is preferred that the UV light reflecting layer comprises particles chosen from the group SiO2, MgF2, Al2O3, MgO, Nb2O5, ZrO2, Ta2O5, CaPO4, LaPO4, YPO4, MgAl2O4 and YBO3, the average particle size lying between 100 nm and 500 nm in each case.
Particles of this composition show no or only a small absorption in the wavelength range from 200 to 400 nm and withstand the high temperatures prevailing during manufacture of a plasma picture screen. In addition, particles having this diameter show a substantially greater light scattering in the UV wavelength range than in the visible wavelength range. This also has the effect that the UV light reflecting layer transmits visible light.
Another favorable embodiment is characterized in that the UV light reflecting layer comprises a layer sequence with layers having a refractive index nxe2x89xa71.7 alternating with layers having a refractive index nxe2x89xa61.5.
In this embodiment, the materials used in the layer sequences are permeable to UV light and visible light. The individual layer thicknesses are chosen such that the UV light is reflected by interference, whereas visible light is optimally transmitted.
It is also preferred that the blue-emitting phosphor is BaMgAl10O17:Eu, the green-emitting phosphor is chosen from the group of Zn2SiO4:Mn and BaMgAl10O17:Eu,Mn, and the red-emitting phosphor is chosen from the group of (Y,Gd)BO3:Eu, Y2O3:Eu, and Y(V,P)O4:Eu in the phosphor layers on the carrier plate.
The blue-, red-, and green-emitting phosphors are chosen in dependence on the emission wavelength of the VUV phosphor used. The application of the VUV phosphors renders it possible to use phosphors in the phosphor layers whose excitation range lies outside the VUV range. Thus, for example, the red-emitting phosphor Y(V,P)O4:Eu may be used instead of the frequently used (Y,Gd)BO3 as the red-emitting phosphor, because the former has a better color point.